The present invention relates to a device for optically determining the position of an object. The device includes a radiation source unit for generating a radiation beam, a radiation-sensitive detection system and an optical imaging system arranged in the radiation path of the radiation beam for forming a radiation spot on the detection system. The optical imaging system is coupled entirely or partly to the object in such a way that the position of the radiation spot on the detection system is a measure of the position of the object in a direction transverse to the radiation path.
A device of this type may be used, inter alia, for determining the position of a freely suspended scanning mirror in a scanning device. The invention therefore also relates to apparatus for recording and/or reading information in an optical record carrier, which device comprises a polygon mirror rotatable about an axis for scanning the record carrier.
In apparatus for optically recording or reading large quantities of information on a record carrier by a scanning spot, this scanning spot should cover a large distance per unit of time on the record carrier. This is necessary because the information density on the record carrier is bounded by the scanning spot dimension which in its turn is determined by the wavelength of the radiation and the numerical aperture of the scanning system. Both magnitudes cannot be chosen arbitrarily. For example, when a television program of HDTV quality is or has been stored in a digitized form on the optical record carrier and if the linear information density on the record carrier is approximately 1.5 bit/.mu.m, the scanning speed of the scanning spot should be approximately 60 m/s. Even when using a limited number of parallel scanning spots, the scanning speed is more than 10 m/s. To reach such a speed using a scanning mirror, for example, a rotating polygon mirror, and to keep the scanning spot accurately aimed at the positions to be scanned, a constant measurement of the polygon or mirror position and feedback to the control mechanism of the mirror is necessary.
Such a measurement is to be preferably performed in a contactless way, for which, according to the invention, a device as described in the opening paragraph can be used.
Such a device is described per se from EP-A-0, 124, 145. This application describes an optical measuring system in which the end of a first optical fiber is imaged by a lens or a concave mirror on a plane in which the ends of the two other optical fibers are located. The first optical fiber is connected to a radiation source and the second other fibers are connected to detectors. The imaging lens or mirror is coupled to a membrane whose displacement results in a displacement of the lens or mirror transversely to the direction of the incident radiation beam. Consequently, the radiation spot which has been formed is also displaced with respect to the ends of the two optical fibers so that the relative radiation intensity on the two detectors changes. In this way the relative intensity received by the detectors is a measure of the position of the object in a direction transverse to the radiation path to which the lens is coupled.
In the conventional device it is not possible to detect the displacement of the imaging system in a direction parallel to the radiation beam. For an accurate determination of the position of an object, such as a polygon mirror or another scanning mirror, it is, however, necessary to measure the position in three dimensions in a rapid and reliable manner with a minimum possible number of additional optical systems. Possible additional optical elements should preferably not be coupled mechanically to the scanning mirror because this would lead to an increase of the mass and the complexity of the system of the scanning mirror.